A STUDY  OF  THE  FACTORS  AFFECT- 
ING THE  ELECTRODE  POTENTIAL 

OF  ZINC 

BY 

ARTHUR  ROWAN  MURPHY 


THESIS 


FOR  THE 


DEGREE  OF  BACHELOR  OF  SCIENCE 

IN 

CHEMICAL  ENGINEERING 


COLLEGE  OF  LIBERAL  ARTS  AND  SCIENCES 


UNIVERSITY  OF  ILLINOIS 

1922 


I 922 
M95 


UNIVERSITY  OF  ILLINOIS 


THIS  IS  TO  CERTIFY  THAT  THE  THESIS  PREPARED  UNDER  MY  SUPERVISION  BY 


_ _ _ Jtowan  Uurph^  _ 


EN titled A_ _Stud^ _ of  _ t he  _ Pac  t or s _A f f _e_c  t ing _ t_h  e_ El  e c t r ode 


Potential  of  Zinc, 


IS  APPROVED  BY  ME  AS  FULFILLING  THIS  PART  OF  THE  REQUIREMENTS  FOR  THE 


DEGREE  OF  


in  Chemical  Engineering, 


Instructor  in  Charge 


Approved  iLUY-' 


ACTING  HEAD  OF  DEPARTMENT  OF  -HHEMXSTIIY-: 


500208 


Digitized  by  the  Internet  Archive 
in  2015 


https://archive.org/details/studyoffactorsafOOmurp 


Iclmowlodgment  - 

Introduction 

Theoretical 

Historical 

experimental 

Conclusion  and  Surarnary- 


paae 

- 1. 

- 2. 


4-14. 


15-23. 


Bibliography- 


■24 


. 


- 


ACMOVh^GdThT 


The  author  v/ishes  to  take  this  opioortunity  in  expressing  his 
sincere  appreciation  and  thanks  to  Doctor  Gerhard  dietrichson,  under 
whose  direction  this  investigation  v/as  carried  out,  and  for  '..hose 
co-operation  and  assistance  he  is  indebted. 


-2- 


Intro Auction 

Inny  methods  have  "been  tried  in  an  effort  to  determine  the  elec- 
tro de  potentials  of  the  more  simple  metals  such  as  zinc  silver,  copper, 
and  cadmium. 

'iiile  most  of  the  investigations  carrie:1  out  up  to  this  time  have 
"been  successful  in  the  determination  of  the  potentials  of  the  metals 
in  contact  with  concentrated  solutions  of  their  ions,  it  has  been  near- 
ly impossible  to  find  a method  which  will  give  results  in  the  more  di- 
lute  solutions. 

The  fundamental  importance  of  electrode  potentials  in  explainin' 
electro-chemical  reactions  led  to  the  experimentation  ana  results 
which  are  given  in  this  article. 

due  to  the  limited  amount  of  time  and  the  many  difficulties  en- 
countered, the  electrode  potential  of  zinc  v/as  the  only  one  considered 
in  this  worh. 


-o- 


Theoretical 

The  eloctrod  potential  represents  the  work  done  in  carrying  unit 
quantity  of  electricity  between  ^electrode  and  electrolyte.  Hie  normal 
electrode  potential  lias  been  defined  as  the  electromotive  force  of  a 
reversible  electrode  of  the  pure  element  against  a solution  in  which 
the  ion  of  the-  element  is  (hypothetical)  molal.  Hie  electrode  -potential 
of  a metal  in  contact  with  a solution  containing  ions  of  that  metal,  is 
dependent  on  the  concentration  of  the  ions  in  solution.  If  the  ion  is 
positive,  the  potential  difference  becomes  more  positive  with  increase 
in  cone  ntration.  If  the  ion  is  negative,  then  the  potential  becomes 
more  negative  or  less  positive  with  increase  in  concentration.  By  say- 
ing /the  potential  becomes  more  positive  or  mors  negative  is  meant  that 
they  are  positive  or  negative  in  relation  to  the  hydrogen  electrode 
which  is  considered  as  zero,  when  the  electrode  values  are  given  in  re- 
f'rence  to  the  hydrogen  scale.  All  values  given  in  this  article  are  con- 
sidered with  reference  to  hydrogen  as  zero,  as  above  stated. 


■Historical 


Many  investigators  have  worked  with  zinc  potentials,  "but  most  of 
those  have  determined  the  potential  of  the  metal  in  contact  with  a sat- 
urated solution  of  the  zinc  salt,  or  in  highly  concentrated  solutions. 

Che  determinations  of  the  potential  in  dilute  solutions  s eras  to  have 
given  rather  indefinite  and  varied  results.  In  spite  of  this  some  very 
interesting  work  lias  been  done  along  this  line,  an'  altho  no  positive 
solution  has  been  derived  at,  the  investigators  have  thrown  light  on 
the  problem  an!  have  indicated  the  way  to  further  investigation. 

A brief  review  of  the  work  already  published  on  this  topic  will 
bo  given.  It  may  be  said  at  this  time  that  the  results  given  in  this 
article  were  more  or  less  obtains:'  by  a method  of  experimentation  ob- 
tained by  a collaboration  of  the  conclusions  of  former  investigators 
and  supplemented  only  by  original  ideas  which  would  furnish  a more 
speedy  method  of  determination,  requiring  only  a limited  amount  of  skill 
and  in  this  way  sacrificing  the  minute  accuracy  of  the  former  investigat- 
ors . 

One  of  the  first  investigations  was  that  of  Richards  an'  Lewis (l). 
Chev  studied  electrodes  of  zinc  amalgams  of  different  concentrations 
and  solid  zinc  electrodes  oppose'  to  zinc  amalgams.  Measurements  were 
made  at  such  temperatures  that  the  amalgams  were  wholly  liquid,  for 
their  partial  solidification  is  an  insiduous  cause  of  error  capable  of 
prodticing  serious  results. 

It  was  noticed  by  Meyer  that  the  e.m.f.  of  a cell  of  this  kind  in- 
creases rapidly  on  standing.  Altho  no  explanation  can  he  given  of  this 
phenomena,  oneraay  prevent  by  using  as  an  electrolyte  a solution  which 
has  remaine'  standing  in  contact  with  the  amalgam  for  several  weeks  be- 
fore being  used.  Mhe  constancy  thus  reached  assures  much  greater  accur- 


-5- 

acy  than  cor.l  otherwise  he  obtained.  Theoretically  the  value  of  the 
anion  of  the  electrolyte  or  the  concentration  of  the  cation  should  be 
without  effect  upon  the  results  of  the  values  obtained  for  cells  of 
the  type  used  in  this  investigation.  This  prediction  was  verified  by- 
experiment.  The  cells  with  electrodes  of  zinc  amalgams  were  less  con- 
stant than  those  with  cadmium.  A tabulation  of  these  results  is  given 
below. 

Zinc  Amalgams. 


Cl 

C2 

Cl 

t. 

-3.  obs. 

1.  calc. 

E 

C2 

m 

X 

1. 

1 

I 

o 

9 

30 

.02890 

.02860 

.0000954 

2. 

1_ 

J 

1 

3 

27 

9 

30 

.02920 

.02860 

.0000964 

3 • 

1 

1 

3 

9. 

rr 

O 

30 

.01425 

.01430 

.0000470 

4. 

1 

1 

9 

27 

rr 

o 

30 

.01515 

.01430 

.0000500 

The  rnea 

.suremont  of 

' the 

contact  potential 

of  solid 

electrodes 

always 

been 

subject  to 

considerable 

uncertainty,  due  to 

the  accidc 

of  crystallization,  condition  of  surface  polarization,  an:  other  un- 
known causes.  It  seemed  possible  that  by  sufficiently  increasing  the 
extent  and  diversity  of  the  surface  an  electrode  might  be  obtained 
whose  surface  and  potential  would  bo  the  mean  of  a large  number  of  diff- 
erent values  and  therefore  constant.  An  electrode  of  this  type  c ns  is  ting 
of  a quantity  of  the  finely  ivided  metal  perhaps  a cm.  in  depth,  pa ch- 
ed  loosely  around  a seale:’  in  platinum  wire  seemed  likely  to  satisfy  the 
desired  conditions,  and  experiments  were  male  with  electrodes  of  this 
sort  and  yielded  remar  dbly  satisfactory  results.  The  difference  in  po- 
1 ntial  between  the  soli  metal  ana  the  amalgam  should  be  emphasized. 


-7- 


It  is  often  stat  i that  the  potential  of  the  saturated  amalgam  may  ho 
considered  the  potential  of  the  true  metal.  In  th  case  of  zinc  this  is 
true  within  a few  thousandths  of  a millivolt.  The  results  obtained  by 
these  men  are  given  here  nd  really  show  a remarkable-  agreement  between 


■-he  potential  of 

the  zinc 

amalgam  and  that 

of  the  zinc  in  thesolii  form 

Z inc  ver s e s malgam . 

cell 

t 

H obs 

• 

E cale. 

1 

30.0 

.01175 

.01160 

.01165 

2 

0.0 

.00570 

.00560 

.00565 

rr 

O 

30.0 

.01170 

.01160 

.01165 

4 

36.2 

.01285 

.01289 

5 

34.5 

.01270 

.01260 

.01255 

6 

32.6 

.01230 

.01215 

.01217 

7 

30.0 

.01170 

.01160 

.01165 

8 

28.0 

.01125 

.01120 

.01125 

9 

26 . 6 

.01095 

.01080 

.01097 

10 

26.7 

• .01100 

.01089 

.01099 

11 

23.  8 

.01045 

.01030 

.01041 

13 

15.7 

.00885 

.00870 

.00679 

14 

16.0 

.00890 

.00675 

.00885 

Frank  J.  He 

11 encamp 

(2)  in  working  on 

the  application  of  the 

Gibb  s -He  c|rriho  1 1 z 

to  concentration 

cells  usee 

1 zinc  amalgams  in  contact 

with  zinc  sulphate  solutions,  end  arrived  at  some  very  interesting  re- 
sults. As  with  the  fault  of  many  former  investigators,  his  work  with  mere 
or  less  concentrated  solutions,  the  most  ilute  electrolyte  use'  being 
one  mol  of  zinc  sulphate  with  four  hundred  moles  of  water  which  is 
approximately  .14]  . 

He  found  that  amalgam  electro  es  are  best  adapted  for  work  on  elec- 


paring  his  zinc  chloride  solutions  will  he  given  here. 

The  zinc  chloride  solutions  were  prepare  ’0/  i luting  a stool:  solu- 
tion of  zinc  chloride.  This  stool:  solution  v/as  prepared  as  follows:  Pure 
hydrogen  chloride  was  proper  ed  by  drop  ing  pure  cone  ntrate'  sulphuric 
acid  slowly  into  pure  concentrated  hydrochloric  acid.  The  gas  was  absorb- 
ed in  conductivity  water  until  the  concentration  was  about  0,5  i.  -in  ex- 
cess of  spongy  zinc,  carefully  washed,  was  then  added  to  this  acid  solu- 
tion and  the  mixture  warmed  gently  until  nonfur ther  zinc  would  dissolve. 
Zeis  solution  re  den  1 litmus  slowly.  The  solution  was  then  filtered  to 
remove  the  excess  zinc  and  the  filtrate  diluted  to  about  0.15  II  nd  employe 
as  a stool:  solution. 

As  has  alrea  y been  stated  finely  divided  zinc  was  used  instead  of 
zinc  amalgam  because  the  amalgam  gave  unsteady  values  of  electromotive 
force.  Between  two  samples  of  the  amalgam  the  electromotive  force  var- 
ied in  an  erratic  meaner  rising  and  falling  rapidly.  The  finely  divided 
zinc  gave  a steady  value,  but  left  something  to  be  desired  in  reproduc- 
ibility in  the  dilute  solutions.  For  this  reason  the  measurements  were 
not  carried  out  in  zinc  chloride  solutions  more  dilu  than  0.0003  1.,  a nd 
in  concentrations  below  0.C01  Id  the  deviation  from  the  mean  is  0.002  volts. 

In  the  table  given  below  are  represented  the  observed  e.m.f.s  of  the 
cell  Zn  ; ZnCl  (c);  ArCl  A . 

In  the  second  table  the  results  given  by  H.  Jahn  are  given  to  supple- 
ment those  given  in  table  2.  Jahn  used  an  amalgamated  zinc  rod  instead 
of  purr  zinc.  'Since  as  pointed  out  b^/  Pichards  and  Lewis  (l),  the  diff- 
erence in  potential  between  pure  zinc  and  its  saturated  amalgam  is  slight 
(only  a few  millivolts ) the  comparison  is  a fair  one  to  make. 

He  found  that  the  normal  electrode  potential  of  zinc  7, as  0.753, 


f 


I 


-10- 


Table  1. 

c.u.f.  of  coll  Jn  : GnCl^c);  J1 
c.  Sn  dl 

moles  per  1000  g.  water. 

0.01021 

0.006022 

0.003112 

0.001453 

0.001253 

0.000772 

0.000649 

0.0003995 

0.0003478 


(observe!  To Its] 

1.1558 

1.1742 

1.1953 

1.2219 

1.2289 

1.2475 

1.2497 

1.2699 

1.2701 


Table  2. 

0.566  1.0306 

1.112  1.0171 

2.220  0.9740 

The  next  work  done  on  the  zinc  electrode  was  that  by  ill ian  G. 

Iloore.  His  work  was  entirely  confined  to  the  use  of  zinc  amalgams . 

In  a study  of  the  literature  ho  found  that  all  previous  investigat- 
ors had  confined  their  attention  euther  to  dilute  zinc  amalgams  of  van— 
in  g composition  and  constant  concentration  of  electrolyte,  or  to  zinc- 
amalgams  or  to  amalgamated  zinc,  with  salt  concentrations  not  less  than 
0.5  IT.  I oreover  in  particularly  all  instances  it  was  found  necessary  to 
take  some  sort  of  precautionto  prcventaccess  of  atmospheric  oxygen  to 
the  electrolyte  and  to  the  electrode. 

Moore  found in  his  work  which  dealt  with  the  electrode  potential  of 
amalgamated  zinc  rods  in  a solution  of  zinc  sulphate  varying  in  concentra8- 
tion  from  0.5  IT  down  to  0.002  IT,  after  considerable  experimentation,  that 
zinc  electrod' s reproducible  at  room  temperature  to  within  one  millivolt 
could  bo  secure-  by  using  well  amalgamated  zinc  rods,  and  by  bubbling  hy- 
drogen thru  the  zinc  half-cell  during  the  mearsurements • 

As  site  . by  him,  I'ellencamp  has  shown  that  with  an  amalgam  contain- 


-11- 

ing  about  1$  sine,  the  temperature  coeffient  of  the  electrode  potential 
of  sine  is  very  small  and  his  onw  work  shows  that  changes  of  several  de- 
grees in  the  temperature  have  only  a slight  effect. 


The  results  obtained  by  him  are  tabulated  below. 

Cf  t.  I 31y  a 1 


Con.  IT. 

mean  obs.  S. 

E calc. 

CX 

0.5  0.5 

-0.784 

-.0780 

30 

0.1 

-0.800 

-.800 

45 

0.01 

—0.823 

-.824 

70 

0.002 

-0.840 

-.850 

85 

As  the  results  of  his  work  he  arrived  at  the  following  conclusions 
and  summary. 

& x rim  e*  f&rs 

1.  Obsrevations  of  previous^ are  confirmed  in  that  it  is  necessary  to 
exclude  oxygon  as  completely  as  possible  in  setting  up  sine  electrodes. 

2.  Electrodes  reproducible  to  within  one  millivolt  can  be  securdd  by 
making  the  dilute  solutions  with  water  recently  boiled  and  cooled  in  hydro- 
gen, the  dilution  taking  place  in  an  atmosphere  of  hydrogen  with  the  meas- 
urements being  made  while  the  hydrogen  is  bubbling  thru. the  electrode  vess- 
el. 

3.  Using  the  precautions  Just  described  the  electrode  potential  of 
sine  in  0.5,  0,1,  0.01,  0.002  II  concentrations  of  zinc  sulphate  have  been 
measured  at  room  temperatures.  Calculated  values  base"  on  the  electrolyt- 
ic dissociation  of  sine  salt  and  these  measurements  are  in  good  agreement 
with  each  other. 

In  a recently  published  article  by  Theodore  Richards  and  Theodore 
dunham  Jr.,  they  have  shown  the  effect  of  changing  hydrogen-ion  concen- 
tration on  the  potential  of  the  zinc  electrode. 

'.Then  a metal  of  weak  basic  character  (such  as  gallium)  is  studied 
with  regard  to  its  single  electrode  potential,  the  salt  solution  in  which 
the  metal  is  immersed  must  be  acidified  to  prevent  hydrolysis.  According- 


-12- 

ly  their  paper  recounts  a brief  s tudyno  f the  effect  of  adding  acid  to  the 
salt  solution  ar<  a typical  reproducible  electrode,  in  order  to  demon- 
strate eirp or imon tally  the  sign  and  magnitude  of  the  potential  difference  thus 
caused. 

Keasur orients  of  the  potential  of  sine  in  acid  solutions  of  sine  sul- 
phate measured  against  the  calomel  electrode  (thru  a sodium  sulphate 

i&ge]  and  also  iirectly  against  sine  el  ctrode  containing  p 0+ 

solution  shorn d marked  rise  in  potential  as  the  acid  concentration  ms 
i .creased. 

Che  magnitude  of  this  rise  in  potential  indicate  that  it  must  he 
due  to  the  liquid  junction  potential  owing  to  tl  the  hydro- 

gen ion. 

Hqual  concentrations  of  t"  sulphate  ion  in  the  form  of  alhali  sul- 
phate add  d to  the  sine  sulphate  solution  gave  smallei  elev  tion 
itential  which  were  in  the  order  of  the  transport  numbers  of  the  cations 
cone  rned.  If  it  is  assumed  that  the  change  in  junction  potential  pro- 
duced byt  sodium  sulphate  is  small,  one  may  infer  that  about  2/3  of  the 
change  produced  by  the  sulphuric  acid  is  due  to  solution  junction  potential 
and  the  other  third  to  a real  increase  in  the  single  electrode  potential 
of  sine,  due  to  , presumably,  t'  e depression  of  the  electrolytic  disso- 
ciation of  the  sine  sulphate. 

Cho  results  as  obtaine  by  these  mon  arc  given. t.ab'1  o 3. 

In  summing  up  the  worh  of  the  for  mentioned,  investigators,  we  arrive 
at  the  following  conclusions  that  have  a direct  baring  on  the  problem  un- 
cons id  .ration. 

1.  d .:  .d.  of  the  typo  of  coll  vn dor  Consideration  increases  rap- 
idly on  standing,  do  is  may  be  prevented  by  using  as  an  electrolyte  a solu- 


-13- 

ticn  which  has  remained  standing  in  contact  with  the  amalgam  for  sever- 
al weeks  "before  using. 

2.  In  the  c;'  sc  of  sine  the  po+ontial  or"  the  saturated  smalg. .mis  * 1 th- 
in a for  millivolts  of  the  potential  of  the  pure  metal. 

3.  .inr.lga.rn  electrodes  arc  host  adopted  for  work  on  electro  do  potentials 
of  sine,  fine  with  pure  metals  it  is  very  difficult  to  prevent  oxidation 

or  to  exclude  occluded  gases  variable  and  unsatisfactory  results  are  ob- 
tained. 

4*  Upon  diluting  zinc  chloride  solutions  hydrolysis  takes  place  caus- 
ing a change  in  concentration. 

5.  fine  sulphate  solutions  giro  more  consist  ts  than  those 

of  zinc  chloride. 

6.  for  work  with  dilute  solutions  it  was  impossible  to  obtain  repro- 
dticibl  results”'! th  a flowing  el'  ctrolytc  . 

7.  Electrodes  reproducible  to  with  in  one  mi?.livolJ'  can  be  secur 
making  the  dilute  solutions  with  water  recently  boiled  an7  cooled  in  hy- 
drogen, the  dilution  taking  place  in  an  atmosphere  of  hydrogen  and  the  meas- 
urements being  made  while  hydogen  is  bubbling  thru  the  electrode  vessel. 

8.  Changes  of  several  degrees  in  temperature  have  only  a slight  effect. 

9.  If  the  electrolyte  is  acidified  the  potential  Is  increased. 

The  last  statement  seems  to  be  in  direct  opposition  to  the  results 
that  were  obtained  by  Korsch,  for  as  was  stated  ‘ efore  all  his  results  were 
obtained  7 " using  a zinc  chloride  solution  that  was  "istinctly  a oil  la 
aracter.  SPhis  would  tend  to  make  his  results  higher  than  the  to ore t- 
ical  values.  It  was  due  to  this  acid  condition  that  he  was  able  to  pre- 
vent hydrolysis  which  otherwise  would  have  led  him  into  many  difficulties. 

It  was  wit!  t3  • conclusions  that  the  experimentation  on  this  rob- 


lom  was  carried  out 


-14- 

rn2-le  5. 


2 k 0,. 

Total  potential 

/ 

IT.  ornel  electro's. 


0.0 

1.0777 

0.1 

1.0328 

0.5 

1.0971 

1.0 

1.1066 

2.0 

1.1175 

4.0 

1.1277 

0.0  again 

1.0771 

T ange  of  Potential. 

1 . i + ■ » ' z + 


"hr  i&ge 

"bridge 

nv. 

irrv. 

0.0 

0.0 

5.1 

6.0 

19.4 

21.6 

28.9 

52.6 

59.8 

45.3 

50.0 

57.5 

( -0.6  ) 

0.0 

-15- 

Experimental 

tthrup  Student  potentiometer  was  us  . ' in  ml  i tl  s«m.  f • 

To  terminations.  A "oston  coll  v/as  used  as  a standard  and  a n/lO  c-lonel 
cell  as  tile  reference  electrode.  A 5H  KC1  solution  was  us'  es  a "buffer 
to  eliminate  liquid  junction  potentials. 

The  first  worli  was  lone  using  an  electrolytic  zinc  amalgam  as  an  elec- 
trode. Very  inconsistent  results  were  obtained,  foe  cause  of  which  was 
later  found  to  be  partially  due  to  the  fact  that  the  amalgams  were  only 
■tially  saturated.  In  these  ie terminations  zinc  sulphate  solutions  o: 

I , Il/lO,  I./lOO,  ll/lOOO  concentrations  v;ero  used.  The  results  obtained  in 
the  1-  andld/lO  concentrations  were  found  to  be  fairly  constant  but  wore  low 
due  to  the  partial  saturation  of  the  zinc  amalgam.  Hie  results  obtained  in 
the  Id/ 100  and  I.l/l000  solutions  were  ridicuously  inerts  is  tent,  at  times, 
the  otential  going  in  the  wrong  direction  upon  diluting  the  electrolyte. 

It  was  thought  advisable  to  try  using  zinc  chloride  instead  of  zinc 
sulphate  as  the  contact  solution.  All  attempts  to  dissolve  chemically  • 
pure  zinc  chloride  in  water  were  unsucessful  due  to  the  high  degree  of 
hydrolysis.  Hie  method  used  by  Borsch  was  then  resorted  to,  that  is, 
dissolving  pure  zinc  in  a solution  of  hydrochloric  acid  of  known  0 f ig^tjL 
Platinum  was  used  as  a catalylizer  in  aiding  in  the  solution  of  the  zinc, 
and  the  solution  heated  to  aid  in  the  reaction.  After  all  reaction  bet":  n 
the  zinc  and  hydrochloric  acid  load  coasod  the  solution  was  tested  with  lit- 
mus and  found  to  be  distinctly  j^pi'..  Hie  excess  zinc  was  then  filtered  off 
and  this  solution  used  as  a stoch  solution,  op  on  diluting  this  solution 
to  IT/10  hydrolysis  to  oh  place,  this  necessitated  re  filtering  the  Il/lO  so- 
lution, consequently  a lowering  of  the  concentration.  As  the  results  de- 
sired were  only  to  be  approximate  this  fact  was  not  tahen  into  consider- 


ation 


-16- 


r'our  mechanic  ly  made  amalgams  were  used  and  were  1,  2,  S,an&  10fb 
zinc  respectively.  The  main  intention  in  using  those  certain  concentrations 
was  to  determine  the  point  at  which  the  amalgans  would  he come  saturated  and 
whether  or  not  that  after  reaching  saturation  the  potential  of  the  amalgams 
woul  remain  constant  with  varying  amounts  of  zinc, tfteso  amonts  h-  ing  a- 
hove  the  saturation  point. 

The  amalgams  were  made  hy  shaking  chemically  pure  zinc  in  contact 
with  mercury  which  load  been  purified  with  nitric  acid  and  lisstilled. 

The  vesults  obtained  with  the  amalgams  in  contact  with  the  Id  and 
Id/  1C  solutions  of  zincchloride  showed  a narked  degree  of  consistency.  They 
also  showed  that  the  electrode  potential  of  the  amalgam  increases  direct- 
ly it'  the  concentration  of  the  zinc  in  the  amalgam  form  up  to  the  point 
of  saturation.  After  the  point  of  saturation  has  be  n reached  the  poten- 


tial of  the  amalgam  remains  constant  with  an  increasing  concentration  of 
the  zinc.  The  results  obtained  for  these  two  concentrations  of  el  ctro- 


lyte  are  given  below. 


Zn 

(amalgam)/  II 

ZtTSO/  3 i 

I KCl/  ca! 

Lomel. 

fo  zinc  in 

amal. 

1. 

2. 

•-  • 

• 

o 

l — i 

e.m.f. 

e.m.f. 

e.m.f. 

e.m.f. 

-1.0925 

-1.0990 

-1.0990 

-1.0907 

1.0927 

1.0989 

1.0985 

1.0980 

1.0925 

1.0990 

1.0988 

1.0985 

1.00925 

1.0992 

1.0987 

1.0985 

1.0925 

1.0992 

1.0987 

1.0985 

1.0925 

1.0980 

1.0987 

1.0S87 

1.0927 

1.0979 

1.0980 

1.0986 

1.0927 

1.0977 

1.0987 

1.0985 

1.0950 

1.0977 

1.0986 

1.0985 

1.0929 

1.0975 

1.090 5 

1.0985 

ave.  e.m.f. 

-1.0920 

-1.0984 

-1.0987 

-1.0980 

E.  P. 

-0.7560 

-0.7024 

-0.7627 

-0.7020 

‘ • • 


■ 


i 


-17- 


2ii  amalgam/  .11 

,j  nine  in  amalgam  1. 

e.m.f . 

-1.1275 
-1.1272 
1.1271 
1.1278 
1 . 1270 
1.1270 
1.1268 
1.1266 
1.1265 
1 . 1262- 

ave.  e.m.f.  -1.1270 
E.  P.  -.7910 


* 

TTC1/  calomel. 

2. 

5. 

10. 

e.m.f. 

e.m.f. 

o .m.f . 

-1.1354 

-1.1352 

-1.1350 

-1.1555 

-1.1350 

-1.1352 

1.1351 

1.1350 

1.1350 

1 . 1547 

1.1350 

1.1351 

1.1345 

1.1348 

1.1353 

1.1350 

1.1356 

1.1352 

1.1347 

1.1352 

1 . 1352 

1.1344 

1 . 1352 

1.1350 

1.1345 

1.1354 

1.1352 

1.1545 

1.1350 

1.1352 

-1.1548 

-1.1351 

-1.1351 

-.7988 

-.7991 

-.7991 

In  plotting  these  results  against  the  concentrations,  it  is  shown 
that  a zinc  amalgam  of  between  2 end  Z%  zinc  is  saturate;  . It  is  also 
shown  that  after  reaching  the  point  of  saturation  the  electrode  potential 
of  the.  metal  remains  constant. 

Erie  results  shown  here  are  by  no  meahs  due  to  the  inexactness  of  the 
concentrations  of  the  solutions.  But  the  relative  values  of  the  potential 
in  I.'  and  I'/lQ  concentrations  have  very  near  the  teoretical  relation  and 
show  that  in  the  more  concentrated  solut'ons  of  zinc  chloride  the  poten- 
tial of  zinc  can  be  determined  with  a fair  degree  of  accuracy,  and  with 
a limited  number  of  precautions. 

These  four  amalgams  were  trie  in  the  zinc  chloride  solutions  of 
h/lOO  and  Id/lOOO  concentrations  but  it  was  impossible  to  obtain  any  con- 
stant or  reproducible  results.  Ehe  met  od  of  using  a flowing  electrolyte 
was  resorted  to  but  seemed  to  have  no  effect  on  giving  more  constant  val- 
ues than  those  obtained  with  a stationary  electrolyte. 

In  making  th  se  de term:' nations  an  electrode  vessel  of  the  type 
shown  in  the  illustration  was  used.  In  using  a flowing  electrolyte 
si  la  1 sep  rat ' i si  d 


funnel  nearly  touching  the  surface  of  the  amalgam. 


The  electrolyte  was 


-18- 


run  into  th  n*  ns, 

tirnos  keeping  the  Surface  of  the  amalgam  in  contact  with  fresh  electrolyte. 

Che  amalgam  use  was  at  all  times  kept  covered  with  a co  c ntrated 
solution  of  the  zinc  salt  which  kept  it  free  from  oxidation.  Just  "before 
being  used  it  was  thoroughly  washed  with  disstilled  water,  and  at  all  times 
kept  out  of  contact  with  the  air.  In  inserting  the  amalgam  into  the  electron  v 
vessel  it  was  x>ipetted  out  of  its  container  with  a one  c.c.  pipette  and 
run  into  the  vessel  already  containing  the  electrolyte.  Chose  pirec-  utions 
wore  taken  to  prevent  undue  oxidation  of  the  electrolyte  and  the  amalgam  • 
by  the  air. 

Since  it  was  impossible  to  obtain  results  with  saturated  amalgam  in 
dilute  solutions,  the  1'Moly  divided  metal  was  tried.  This  metal  was  ob- 
tained by  electrolytic  deposition  from  its  salt  solution  and  was  obtained 
in  its  spongy  form.  As  in  the  case  of  tho  saturated  amrlgam  goo  results 
were  obtained  in  the  M, M/lO  zinc  chloride  solutions  but  in  the  more  di- 
lute solutions  the  same  difficulties  were  encountered  as  in  the  previous 
experiments. 

It  7/as  thought  at  this  time  advisable  to  check  up  the  electrolytic 
zinc  with  the  concentrated  amalgam.  As  the  results  given  in  table  4.  show, 
the  potential  of  the  spGngy  metal  an  ’ the  saturated  amalgam  agree  within 
0.4  millivolt  this  is  close  enough  for  the  purpose  for  which  this  invest- 
igation vas  carried  out.  It  shpws  that  it  is  perfectly  legitimate  to  use 
the  saturated  amalgam  in  place  of  the  pure  metal.  In  fact,  it  is  highly 
advisable  for  it  is  much,  easier  to  get  a good  contact  and  the  readings 
are  much  more  constant  with  the  amalgam.  It  is  easier  to  handle,  and  the 
surface  strains  are  eliminated. 


-19- 


"'loctrolytic  nine 
zinc  amalgam 


J ' ~ X: 


electrolytic 

amalgam. 


3c.tu.rat  T amalgam 


e.m.f . 

-1.1153 

1.1155 

1.1158 
1.1155 

1.1159 

1.1160 

1.1157 

1.1158 

1.1160 
1.115 


e.m.  f. 

-1.1156 

1.1154 

1.1155 
1.1153 
1.1153 

1.1153 

1.1154 

1.1152 

1.1153 

1.1155 


Ave  -1.1157 


-1.1153 


Since  it  was  impossible  to  obtain  reproducible  results  of  the  potential 
of  saturated  amalgams  in  contact  with  th  dilute  zinc  chloride  solutions 
it  ms  thought  that  this  may  have  been  due  to  the  extensive  hydrolysis 
of  zinc  chloride  in  solution.  It  was  thought  advisable  to  make  one  more 
attempt  with  zinc  sulphate.  As  before  very  goo’  results  w-  rr  obtained 
in  the  I and  Y / 10  concentrations  but  beyond  this  dilution  the  results 
v/ore  not  constant, altho,  much  improvement  could  be  sex  over  those  ob- 
tained in  the  zinc  chlori’e  solutions. 

In  taking  readings  in  the  dilute  solutions  much  difficulty  was  en- 
countered due  to  th' ir  low  c nductivity.  A moans  of  remedying  this  trouble 
v/as  tried  an  showed  that  the  method  had  many  possibilities.  Since  the  t 
trouble  under  consideration  v/as  due  to  the  low  conductivity  of  the  solu- 
tions, the  means  of  elininat'ng  this  consisted  in  addins  some  inert  salt 
to  the  solution  which  would  incr- use  the  conductivity  and  not  effect  the 
ionization  of  the  zinc  sulphate.  .1  IT  was  aided  to  the  IT  zinc  sulphate 
to  increase  its  conductivity.  Four  cells  were  tried  containing-  0.0,  5.0, 
10.0,  an"  15.0  c.c.  of  .1  II  KOI  per  50  c.c.  M zinc  sulphate  respective- 
ly. Checks  on  these  colls  wore  run  and  pur  lisstill  i water  was  used 


-20- 


in  place  of  the  TCI  J-n  le  to  mine  7/hether  or  not  the  KOI  had  any  effect 
on  the  potential.  The  results  obtained  are  shown  below. 

.1  r TC\  in  5C  c.c.  !*  TnSO*/  lei. 


. 

o 

. 

o 

a 

o 

rH 

a 

r 1 

• 

5 c . C'. 

10  c.  c. 

15  c.c. 

e.m.f . 

e.r.f. 

e.m.f. 

e.m.f, 

-1.1184 

n 

•>  J.  £.■ 

-1.1216 

-1.1240 

1.1180 

1.1200 

1.1215 

1.1238 

1.1183 

1.1205 

1.1221 

1.1235 

Ave.  -1.1182 

-1.1204 

-1.1217 

-1.1238 

Diff.  in  m.m. 

2.2 

1.3 

2.1 

Duplicate  cell  using 

7/a  ter 

ins tea ■ 

of  . 

1 IT  KGJ. 

1.1194 

1.1215 

1.1235 

1.1194 

1.1214 

1.1235 

1.1194 

1.1214 

1.1231 

Ave.  -1.1194 

-1.1215 

-1.1234 

Diff.  in  m.m. 

2.1 

1.9 

These  results  are  hardly  general  enough  to  draw  any  conclusions 
from,  hut  at  least  show  that  this  proceedure  has  possibilities.  They  al- 
so do  not  show  us  the  effects  in  dilut  solutions.  The  determination  of 
these  effects  would  constitute  a problem  in  itself.  Due  the  limit: a- 
mount  of  time  no  mere  work  was  'one  along  this  line  except  to  make  an 
attempt  at  using  the  KOI  in  the  two  dilute  concentrations.  It  was  found 
th  t more  accurate  readings  could  be  obtained  but  as  before  the  ree finds 
varied. 

In  the  first  part  of  this  article  the  r-  suits  obtained  by  Ido  ore  and 
the  conclusions  arrived  at  by  him  were  discussed.  The  method  used  by  him 
next  tried.  This  method  seems  to  have  great  possibilities  and  with 
the  modifications  given  gave  very  satisfactory  results. 

Tiie  results  in  the  dilute  solutions  wcr~  obtained  by  using  a cell 
which  hertofore  load  been  used  as  a hydrogen  electrode  and  seems  to  be  the 
mosi  satisfactory  tj’pe  that  could  be  used  with  this  method.  The  solutions 
v/ere  placed  in  this  cell  and  thoroughly  saturated  with,  hydrogen  before 
inserting  the  am&lgam.  The  flow  of  hydrogen  thro  th:  electrolyte  7/as  con- 


-21- 


tinued  during:  the  entire  proc  ss  of  inserting  the  amalgam  and  while  the 
measurements  were  b ing  made.  One  of  the  most  striking  effects  of  tais 
method  is  that  the  surface  of  the  amalgam  remains  bright  four  hours, 
while  in  the  other  methods  tried  the  surface  of  the  amalgam  became  coat- 
ee with  a grayish  white  precipitate  after  a very  short  time.  From  this 
effect  it  seems  that  the  large  deviation  of  results  obtained  in  the  dilu  te 
solutions,  without  being  at  all  times  saturated  with  hyt  rogen,  7/as  due  to 
the  hydrolysis  of  the  electrolyte  or  the  oxidation  of  the  surface  of  the 
amalgam  by-  the  dilute  electrolyte. 

file  results  shown  here  for  the  potentials  in  the  M an'  Y.J  10  solutions 
were  obtained  without  the  use  o f hydrogen  since  those  results  could  be 
obtained  without  any  difficulty  an  with  an  exactness  which  was  sufficient 
for  this  purpose. 

©lose  given  for  the  M/lOOand  I'/lOOO  solutions  were  obtained  by  bubb- 
ling hydrogen  thru  the  electrolyte  in  the  manner  before  described. 


Zn  amalgam  / M Zn 30f  / 3N  KOI  / cal oi 
1.  time  2.  time  3.  time 


-1.1280 

1/45  -1.1277 

1/45 

-1.1269 

1/51 

1.1280 

1/54  1.1272 

1/55 

1.1272 

1/55 

1.1276 

1/57  1.1272 

1/58 

1.1271 

1/59 

1.1275 

2/05  1.1265 

2/08 

1.1272 

2/10 

1.1270 

2/11  1.1266 

2/12 

1.1272 

E/16 

1.1270 

2/17  1.1264 

2/18 

1.1269 

2/20 

-1.1275 

-1.1270 

1.1271 

Zn 

aralgam  /O.llT  \ 

5nS0#  / 3 I KOI 

/ calomel 

-1.1505 

9/45  -1.1497 

9/56 

-1.1494 

9/54 

1.1507 

10/00  1.1499  10/49 

1.1497  10/52 

-1.1506 

-1.1498 

-1.1496 

Zn  amalgam  / o.Ol  U Zn 30*  / 3 IT  KOI  I calomel 
-1.1679  9/13  -1.1679  6/07  -1.1678  5/57 

-1.1679  9/25  -1.1682  9/21  -1.1680  9/l6 

1.1678  1/31  1.1679  1/28  1.1674  l/34 

Ave.  -1.1679  -1.1680  -1.1677 


-22- 


Zn  amalgam/  0.001  r ZnoO*.  / 3 TT  EU1  / calomel. 


1. 

time 

2. 

time 

-1.1923 

l/46 

-1.1919 

1/45 

1.1924 

1/59 

1.1922 

1/58 

1.1927 

2/14 

1.1927 

2/12 

1.1931 

3/45 

1.1935 

3/43 

1.1930 

5/16 

1.1933 

4/26 

1.1930 

5/41 

1.1939 

5/40 

Ave. 

-1.1928 

-1.1930 

Average 

Results. 

con. 

Difference 

electrolyte 

e.n.f. 

2.  potential 

m.m. 

M. 

-1.1272 

-0.7912 

0.1  M. 

-1.1500 

-0.8 140 

23 

0.01  M. 

-1.1679 

-0.8320 

18 

0.001  11. 

-1.1929 

-0.8570 

25 

If  the  results  obtained  are  plotted  v/ith  those  obtained  by  Iloore 
( as  in  the  grapfe  ),  it  is  seen  that  altho  the  results  are  not  absolute- 
ly the  same,  due  to  only  a necessary  amount  of  precaution  in  the  experi- 
mental work,  they  have  nearly  the  same  relation.  Ibis  prooves  the  unlim- 
ited possibilities  of  this  method  of  determination  and  opens  a large  field 
of  investigation. 


-L3- 

Conclusior.  and  Summary. 

1. 

concentration  du  to  the  high  degree  of  hydrolysis. 

2.  The  electrode  potential  of  zinc  amalgam  increas  s directly  wit' 
tl  tr  concentration  of  the  zinc  in  the  amalgam  form  up  to  the  point  of  sat- 
uration. After  the  point  of  saturation  hcs  "been  reached  the  potential  of 
the  araclgam  remains  constant  with  an  increasing  concentration  of  the  zinc. 

5.  A saturated  zinc  amalgam  contains  between  £ and  3 fo  zinc. 

A.  In the  more  concentrated  solutions  of  zinc  chloride  the  potential 
of  zinc  can  "be  det  rain  d with  a fc  ir  degree  of  accuracy  and  with  a lim- 
ited n'  mber  of  precautions. 

5*  It  was  impossible  to  obtain  results  with  the  saturated  amalgams 
in  dilute  solutions  even  when  resorting  to  r.  flowing  electrolyte. 

6.  The  potential  of  the  spongy  metal  and  the  saturated  amalgam  a- 
gree  within  0.4  millivolt. 

7.  The  addition  of  IIC1  to  the  electrolyte  to  increase  the  conductiv- 
ity Was  shown  to  have  no  effect  on  the  potential  in  the  concentrated  so- 
lutions. Limited  time  prevented  its  trial  in  the  dilute  solutions. 

8.  Very  good  results  werr  obtained  in  all  dilutions  by  using  a zinc 
Sulphate  solution  saturated  with  hydrogen  and  bubbling  a continuous  stream 
of  hydrogen  thru  the  electrolyte  while  the  reelings  arr  being  tahen • 


I 


; 


-24- 


Bibliography. 

1.  Hi  char  els  and  Lewis.  Proc.  Amer.  Acad. , 34-,  98  (1898) 

2.  lie  11  encamp,  Phys.  Lev.,  29,  329  (1909) 

3.  lloore,  J.  A.  C.  S.,  43,  81  (1921) 

4.  Theodore  7.  Richards  and  Theodore  Lunham,  Jr.,  J.  A.  0.  S.,  44,  678(1922) 

5.  Horsch,  J.  A.  G.  S.,  41,  1787  (1$19) 


' 


